In many optical systems, such as projection exposure apparatuses for semiconductor lithography, for example, the electromagnetic radiation used for exposure is also absorbed as an undesirable effect—alongside the desirable refraction or reflection—in the optical elements used, such as lenses or mirrors, for example. The power absorbed in the process often leads to a generally inhomogeneous heating of the optical elements. As a consequence of the temperature-induced changes in the refractive index, expansions and mechanical stresses, the optical system is disturbed, which leads to aberrations of the wavefront propagating in the optical system and thus to an impairment of the imaging quality.
FIG. 1 illustrates an example of a prior art projection exposure apparatus 1 for semiconductor lithography. This apparatus serves for the exposure of structures onto a substrate coated with photosensitive materials, which substrate is generally predominantly composed of silicon and is referred to as wafer 2, for the production of semiconductor components, such as, for example, computer chips.
In this case, the projection exposure apparatus 1 includes an illumination device 3, a device 4 for receiving and exactly positioning a mask provided with a structure, a so-called reticle 5, which is used to determine the later structures on the wafer 2, a device 6 for the mounting, movement and exact positioning of precisely the wafer 2, and an imaging device, namely a projection objective 7, with a plurality of optical elements 8 which are mounted via mounts 9 in an objective housing 10 of the projection objective 7.
In this case, the basic functional principle provides for imaging the structures introduced into the reticle 5 onto the wafer 2. After exposure has been effected, the wafer 2 is moved further in the arrow direction, such that a multiplicity of individual fields, each having the structure prescribed by the reticle 5, are exposed on the same wafer 2. On account of the step-by-step advancing movement of the wafer 2 in the projection exposure apparatus 1, the latter is often also referred to as a stepper. In order to improve the process parameters, in the step-and-scan systems in this case the reticle 5 is continuously scanned through a slotted diaphragm.
The illumination device 3 provides a projection beam 11 used for the imaging of the reticle 5 on the wafer 2, for example light or a similar electromagnetic radiation. A laser or the like can be used as a source for this radiation. The radiation is shaped in the illumination device 3 via optical elements in such a way that the projection beam 11, upon impinging on the reticle 5, has the desired properties with regard to diameter, polarization, shape of the wavefront and the like.
Via the projection beam 11, an image of the reticle 5 is generated and transferred to the wafer 2 correspondingly by the projection objective 7 as has already been explained above. The projection objective 7 has a multiplicity of individual refractive, diffractive and/or reflective optical elements such as e.g. lenses, mirrors, prisms, terminating plates and the like.
The step-and-scan systems described above usually exhibit a scanner slot formed in approximately rectangular fashion, which has the effect that the optical conditions in the scanning direction and perpendicularly thereto are different. This symmetry breaking in the field leads to second-order intensity distributions and thus to second-order disturbances in the vicinity of field planes of the system, that is to say usually on optical elements in the vicinity of the wafer 2 and the reticle 5. In this case, the expression “nth-order” intensity distributions is understood to mean distributions which have a symmetry such that they are transformed into themselves upon a rotation through 360°/n, where n represents a natural number.
This can result in astigmatic image aberrations whose field distribution often contains considerable constant, but also quadratic components. At the same time, moreover, specific field distributions of further aberrations are induced. So-called anamorphism in the case of distortion shall be mentioned here as the most important example. The effects caused by the symmetry breaking in the field have almost always the same sign and similar relationships for a large class of settings, since the intensity distribution brought about by the scanner slot on the lenses near the field is relatively independent of the settings used.
The angular distribution of the illumination setting and also the diffraction effects at the reticle determine the symmetry of the angular distribution of the electromagnetic radiation used. This angular distribution translates into a corresponding intensity distribution and thus into a temperature distribution of the same symmetry in the optical elements near the pupil.
Let us consider the following example for elucidating the disclosure:
During the design phase of a projection exposure apparatus and in particular when selecting the manipulators, illumination settings with very specific symmetries might not yet have been taken into account, for example. In this case, only disturbances with second-order symmetry which originate from the abovementioned symmetry breaking of the rectangular scanner slot are typically considered during the design. As a consequence, only a manipulator which is positioned near the field (where the aforementioned disturbances arise) and which, e.g., by compensating second-order disturbance, e.g. a deformation (in the preferred direction prescribed by the orientation of the scanner slot), corrects the astigmatic image aberrations and the anamorphism in the “correct” (more or less universal) relationship is provided during the design phase. FIG. 1 illustrates such a manipulator with the reference symbol 8′ way of example.
Examples of the use of manipulators are found in the prior art in particular in EP0851304 A2 and also in JP10142555.
For the case where the desired properties made of the projection exposure apparatus change over the course of time, the above-described design of the system proves to be inadequate, however. Thus, it can already be foreseen at the present time that the main emphasis of the applications in the case of many semiconductor manufacturers is shifting to different products than originally planned, for example to the production of flash memories. In order to increase the resolution, a dipole illumination distinguished by two localized poles in the pupil can be used in such applications. Dipoles in the x direction or in the y direction are the most common, as illustrated in FIG. 2 by way of example in subfigures 2a and 2b. In this case, FIG. 2a shows by way of example a so-called x dipole, while FIG. 2b shows a y dipole.
This additional symmetry breaking in the pupil leads, particularly in lenses near the pupil, to a linear combination of greatly second-, fourth-, sixth- and possibly even higher-order temperature distributions. Moreover, the applications tend toward ever more extreme dipoles with ever smaller aperture angles (less than 25°) and ever smaller ring widths of the poles (down to Δσ<<0.1).
In contrast to the effects near the field which are caused by the scanner slot, in this example the symmetry breaking in the pupil leads to symmetry-breaking lens heating (LH) effects in the lenses near the pupil, where it causes an additional astigmatism offset which can have both signs (depending on the orientation of the dipole or the structures to be imaged). At the same time (depending on the aperture angle of the dipole) constant higher-order (e.g. fourth-order, sixth-order, etc.) image aberrations are also induced.
If—as discussed in this example—a (unidirectional) manipulator for the compensation of second-order disturbances is only positioned near the field and no additional manipulator exists in proximity to the pupil, which would actually be involved in order to correct the additional dipole-induced aberrations (depending on orientation X and Y in both directions) at a suitable position (near the pupil), this leads to the following problems:                Although the manipulator near the field can also concomitantly correct a certain portion of the astigmatism offset of the X dipole, nevertheless the anamorphism is then overcompensated for (considerably under certain circumstances) and thus set parasitically by the manipulator near the field. Other parasitic image aberrations also reduce the correction potential of the element near the field.        In the case of the Y dipole, the lens heating induced astigmatism component from the pupil overcompensates for the astigmatism component from the field. Overall, astigmatism is established with a sign that cannot be corrected by the unidirectional manipulator. However, even if the manipulator near the field were bidirectional, a considerable anamorphism (and other image aberrations) would again be established parasitically.        
In the example considered here, therefore, an additional manipulator would be involved in the vicinity of the pupil, which additional manipulator can compensate for the second-order (and possibly also higher-order) disturbances of the lenses near the pupil in both directions. A possible position of such a manipulator is indicated by the manipulator with the reference symbol 8″ in FIG. 1.